1. Field of the Invention
The invention is based on known devices for metering liquid reducing agents or other pollution-reducing liquids in an exhaust system. Such devices are used for instance in the automotive industry, for reducing pollutants in an exhaust gas from an internal combustion engine. The invention furthermore relates to a method for operating the device for metering the liquid reducing agent.
2. Description of the Prior Art
In internal combustion engines, especially diesel-fueled internal combustion engines, because of the more-stringent exhaust gas legislation that will be enacted in the next few years, among other things the proportion of nitrogen oxides in the exhaust gas among will have to be reduced. For reducing the proportion of nitrogen oxide in the exhaust gas, selective catalytic reduction (SCR) is for instance performed, in which he nitrogen oxides are reduced, with the aid of reducing agents, to nitrogen and water. As reducing agents, an aqueous urea solution is used, for example. Still other methods for chemical reduction of pollutants are also known.
In SCR, the reducing agent is typically stored in a tank and is fed from the tank to the metering module via a line. The reducing agent is injected by the metering module, for instance into an exhaust pipe, for instance in the vicinity of a catalytic converter.
One such mixture-dispensing device for introducing a reducing agent into an exhaust system of an internal combustion engine is described in European patent disclosure EP 0 928 884 A2. In it, a urea-water solution is introduced upstream of a catalytic converter by a pump, under pressure and with the addition of compressed air, in order to effect the reduction of nitrogen oxides in the exhaust gas. Besides a reducing agent usage tank, the device shown also has a reducing agent supply tank, from which the reducing agent usage tank can be replenished with urea-water solution via a line equipped with a check valve.
Given the severe ambient conditions under which the systems have to be used in practice, however, the known systems face numerous challenges, some of which so far require great system complexity. One problem, for instance, is that in many eases transferring the reducing agent by pumping entails major effort and expense. For instance, if a pumping device is used not only for metering but also for transferring liquid reducing agent between the storage tank and the working tank by pumping, then this transfer by pumping is possible only either by means of complex control or during after-running of the system, or in other words when the vehicle has been parked. This operation takes several minutes, and in many cases it is very noisy.
Moreover, error recognition in the sense of so-called on-board diagnosis (OBD), which by now is typically demanded, also presents the systems with major challenges. Especially the communication between the storage tank and the working tank must be monitored, which is not always simple when the metering loop and the transfer pumping loop are coupled. Typically, the transfer pumping loop includes valves that can be monitored only with difficulty, so that in the normal operating mode, if such valves leak, overfilling of the storage tank can ensue.
Another challenge is particularly the temperature range in which the systems are typically used. Specifications require operating safety of the systems down to temperatures of approximately −40° C. However, depending on the embodiment of the reducing agent and in particular on the addition of antifreeze agents, the freezing point of the liquid reducing agents (a urea-water solution, for example) is far above that temperature, for instance being approximately −11° C. Accordingly, the systems must be designed to be resistant to ice pressure, which in known systems is usually feasible only by mean of complex provisions for heating the lines. For instance, the connecting line between the storage tank and the working tank must as a rule be embodied in heated form. Valves that are received in that line must as a rule be chosen to be resistant to ice pressure, which is not possible in all cases except at major technological effort and expense.